The present invention relates to an Si base substrate covered by either a CdTe layer or a Cd-rich CdZnTe layer and a method for forming the same.
In the prior art, a CdZnTe substrate has been used for growth of HgCdTe substrate thereon. Notwithstanding, an Si base substrate covered by either a CdTe layer or a Cd-rich CdZnTe layer is useful as a substrate for growth of HgCdTe crystal thereon or for an infrared device due to its low cost, large area and high solidity. The latter substrate will hereinafter be referred as an Si base substrate.
The Si base substrate may be formed by initially forming a buffer layer on a silicon substrate and subsequent growth of a CdTe or CdZnTe thin film thereon. In view of increase of the productivity, reduction of the cost, and high quality due to reduced impurities, it is advantageous to directly form a CdTe or CdZnTe thin film on the Si substrate.
One of the conventional techniques of direct growth of the CdTe layer on the Si substrate is disclosed in Journal of Electronic Materials Vol. 22, 1993, pp. 951-957. It is disclosed to grow CdTe(111)B on Misoriented Si(001) by molecular beam epitaxy. As illustrated in FIG. 1, a single domain CdTe(111)B just or off has been grown on an Si(001) substrate 18 just or off. The full width at half maximum (FWHM) of x-ray double crystal rocking curves (DCRC) measured is 140 arc-s, which represents the quality of crystallization.
A second conventional technique of direct growth of the CdTe layer on the Si substrate is disclosed in Journal of Vacuum Science and Technologies B10(4), 1992, pp. 1370-1375. It is disclosed to grow a (111) CdTe layer on an Si substrate by hot wall epitaxy. The full width at half maximum (FWHM) of x-ray double crystal rocking curves (DCRC) measured is 315 arc-s at a thickness of 6.1 micrometers.
A third conventional technique is a direct molecular beam epitaxy growth of ZnTe(100) and CdZnTe(100)ZnTe(100) on Si(100) substrate which is disclosed in Applied Physics Letters, 63(6) Aug. 1993, pp. 818-820. Structures are illustrated in FIG. 2A, 2B and 2C. As illustrated in FIG. 2A, an intermediate ZnTe(100) layer 20 is a buffer layer with a thickness of 1 micrometer. A (001) CdZnTe layer 21 with a thickness of 11.5 micrometers is formed on the buffer layer 20. The buffer layer 20 is formed on an Si(001)just or off substrate 18. The molecular beam epitaxy is carried out using CdTe, ZnTe, Cd, and Zn. The (001) CdZnTe layer 21 is measured in the full width at half maximum (FWHM) of x-ray double crystal rocking curves (DCRC) at 158 arc-s. Further, as illustrated in FIG. 2B, an intermediate ZnTe(100) layer 20 is a buffer layer with a thickness of 1 micrometer. A CdTe(112)off layer 19 with a thickness of 9.5 micrometers is formed on the buffer layer 20. The buffer layer 20 is formed on an Si(112)off substrate 18. The CdTe(112)off layer 19 is measured in the full width at half maximum (FWHM) of x-ray double crystal rocking curves (DCRC) at 670 arc-s. Furthermore, as illustrated in FIG. 2C, an intermediate ZnTe(100) layer 20 is a buffer layer with a thickness of 1 micrometer. A CdTe(552)off layer 19 with a thickness of 9.5 micrometers is formed on the buffer layer 20. The buffer layer 20 is formed on an Si(112)off substrate 18. The CdTe(552)off layer 19 is measured in the full width at half maximum (FWHM) of x-ray double crystal rocking curves (DCRC) at 110 arc-s.
As described above, the CdTe substrate or the CdZnTe substrate has often been used for growth of the HgCdTe crystal layer thereon, where HgCdTe crystal is a material for infrared-ray detectors. In order to obtain the HgCdTe monocrystal growth, it is extremely important not only to strictly optimize growth conditions but also optimize substrate orientations. If the HgCdTe growth is carried out by molecular beam epitaxy, the (112)B plane is the optimum orientation. This is disclosed in Journal of Crystal Growth 117 (1992) 171-176.
On the other hand, it is difficult to obtain twin-free HgCdTe crystal growth by molecular beam epitaxy overlying an Si(001) substrate. It will be necessary for obtaining the mono-crystal in this orientation that a substrate temperature is precisely maintained within a small variation of .+-.1.degree..degree.C. and Hg flux is also precisely maintained within a small variation of .+-.2%. This is disclosed in Journal of Vacuum Science and Technologies 1990, Vol. A8 pp. 1013-1019. However, it is in fact difficult to keep satisfying these conditions during the HgCdTe epi-growth.
If the HgCdTe growth is carried out on a CdZnTe(001) layer overlying a ZnTe layer which overlies the Si(001) substrate, it is difficult to suppress twin-related hillock formations.
If the CdTe layer is grown on a ZnTe buffer layer overlying an Si(112) off substrate, the CdTe layer grown has double domains with different orientations. Particularly when a CdTe(112) off layer is grown, twin crystal is partially formed. Further, it is difficult to obtain the required put ZnTe growth.
In the foregoing circumstances, there has been no Si base substrate free from the above disadvantages.